1. Field of the Invention
This invention relates generally to apparatus for determining the refractive correction for a patient's eye and relates more particularly to apparatus for simultaneous presentation of a plurality of optical corrections from which the patient may choose.
2. Discussion of the Prior Art
It is common practice in determining the refractive correction for a patient's eye to change the refractive correction by small amounts while asking a question such as "Which is better, 1 or 2?" To facilitate comparison of the two choices, several optical systems have been devised which produce a double image of a distant target, one of the two images being seen through one trial refractive correction, and the other of the two images being seen through a different trial refractive correction. The patient simply indicates which of the two images is more clear, and appropriate changes are made in the overall refractive correction for the eye until the final correction is obtained.
A major problem in comparing two different refractive corrections by viewing a single test target through each is that different magnifications are characteristically obtained through the two different corrections. With the small test targets which must be used for critical testing, differences in magnification are easily mistaken for small differences in blur, leading to error in the determination of the refractive correction.
U.S. Pat. No. 3,240,548 describes a doubling optical system for refractive testing wherein magnification differences are compensated for by careful choice of the lens components through which the two images are viewed. Similar magnification compensation by special lens design is described by Haynes, P.R., "A Homokonic Cross Cylinder for Refractive Procedures," Amer. J. Optom., Vol. 34, pp. 478-485 (1957). A problem is that special lens designs may produce differences in brightness of the two images, leading to error in choosing. This problem is discussed in U.S. Pat. No. 3,811,756, and by Luneburg, R. J., "Modified Simultantest--Validity," Optical J. and Rev. Optom., Vol. 108, No. 2, pp. 29-31 (Jan. 15, 1971). Further problems with special lens designs are complexity and cost, especially when short testing distances are used.
An alternate method for simultaneous presentation of two different trial refractive corrections is to provide two similar test targets in the patient's field of view and to place different lenses in front of each. This method is highly unusual, being described only by Asher, H. "A New Cross Cylinder Method," Oregon Optometrist, Vol. 34, No. 4, pp. 9-11 (1967). Again, magnification differences may be compensated for by special lens design, but an additional problem with this arrangement is that it may only be used with long testing distances.
Accordingly, it is an object of this invention, in systems wherein a plurality of test targets are viewed simultaneously, to provide an alternate and simpler method of compensating for magnification differences.
A further object of the invention is to provide a method of magnification compensation which is applicable to a variety of testing distances.
Other objects and advantages of the invention will become apparent from the following description.